As it turns out, decalcomania is a mania -- or at least it used
to be! Alert linguaphile Mike Lake from uiuc.edu informed:

Uh, it was some kind of mania. Apparently it was all the
rage in the 1860's in Europe. From decalquer (to copy)
and mania (madness, craze).

Noland D., yet another alert linguaphile, whose mail carries the
stamp of uncwil.edu, reported, "It IS a mania; OEDI's entry
implies a mid-19th century French one."

It seems the word touched a creative vein in the subscribers.
Stanton McCandlish at eff.org guessed it was obsessive sticker
collecting. Milind R. Agarwal from msu.edu figured it was "something
to do with a person who was afraid of breaking his bones, due to
poor calcium levels in the body. And Michael G. Koopman of ctc.com
suggested:

I'd take this opportunity to introduce a very creative subscriber
in our list. Debbie Burhans from buffalo.edu uses the word from
A.Word.A.Day to write a limerick -- kind of A.Limerick.A.Day. (-:
Here is what she penned for today:

If you're into decalcomania
When you tell folks they say, please explania
Does it make you depressed
To be decal obsessed?
And they look at you like you're insania

Finally, all of you with raised hands may put them down. you were
right after all. Or used to be.